Currently, there are known blistering machines, which form blisters in a continuous band of heat-formable material and subsequently fill the blisters with products; the blister band, filled in this way with the products, is subsequently closed by welding a sealing band and then cut into single blister packs.
In particular, the band of heat-formable material is unwound from a bobbin and is driven along a horizontal plane between opposite plates of a heating station, and then it is fed through a forming station. In the forming station, suitable forming means act on the band, which has become deformable due to the heat action, so that blisters are formed for subsequent receiving of the products.
The blistering machines currently used are affected by a big problem deriving from the lack of planarity in the obtained blister packs, i.e. from undesired curving, mainly in the longitudinal direction, which the blister packs assume at the outlet of the cutting stations of the blistering machines.
The above problem derives from the expansion of the band of heat-formable material due to heating thereof necessary to form the blisters and to subsequent shrinkage of the band due to the return to the environment temperature.
The above-mentioned shrinkage or contraction of the heat-formable band is not uniform and causes considerable surface tensions of the band which provoke uncontrolled stretches, increased by the subsequent application of the sealing band and consequently determining the curving effect.
In particular, this damaging effect has been reported in case of blister band produced from heat-formable material such as polypropylene.
It is to be noted that the shrinkage of the above-mentioned band, and consequently, the curving effect of the blister packs obtained therefrom, lasts for a long time, with negative results affecting also the subsequent production steps, out of the blistering machine, e.g. during piling and/or subsequent packaging blister packs into the related boxes.
Moreover, the blister band uncontrolled stretching effect is also aesthetically unpleasant, specially if the not perfectly flat blister packs obtained from the above blister band, are put on sale separately instead of being closed inside a box or package.